There is an operation pedal apparatus that includes: a transmitting member including a pedal aria of an operation pedal that is disposed so as to be pivotable about a support axis and subjected to a depressing operation; and an output member to which a depressing operation force is transmitted from the transmitting member and to which a reaction force corresponding to the depressing operation force is applied. For the operation pedal apparatus, a pedal operation amount detection apparatus that includes a sensor member disposed on a load transmission path of the transmitting member and deformed by the depressing operation force and the reaction force and that electrically detects deformation of the sensor member is proposed as an apparatus that detects a depressing operation force or a depression stroke of, for example, a brake pedal for a service brake, a brake pedal for a parking brake, an accelerator pedal, or a clutch pedal in a vehicle.